Coating of webs by freeze-drying and products therefrom

ABSTRACT

COATED PAPER IS PRODUCED BY APPLYING A COMPOSITION COMPRISING A POLYMERIC MATERIAL WITH OR WITHOUT ADDED PIGMENTS DISPERSED IN A LIQUID VEHICLE, FREEZING THE COATING, SUBJECTING THE FROZEN COATING TO A VACUUM TO REMOVE THE MAJORITY OF THE LIQUID VEHICLE, AND RETURNING THE DRIED OR PARTIALLY DRIED COATING ON THE WEB TO AMBIENT TEMPERATURE.

Nov. 14, 1972 T. A. FADNER ETAL 3,702,779

COATING OF WEBS BY FREEZE-DRYING AND PRODUCTS THEREFROM Filed April 21,1970 Li I OOOOOOOOO INVENTQRS THOMAS A. FADNER KARL V- KRASKE flhad 7 fI TORNEYS United States Patent 3,702,779 COATING OF WEBS BYFREEZE-DRYING AND PRODUCTS THEREFROM Thomas A. Fadner, Oxford County,Maine, and Karl V.

Kraske, South Hadley, Mass, assignors to Ethyl Corporatiou, Richmond,Va. Continuation-impart of application Ser. No. 749,008,

July 31, 1968. This application Apr. 21,1970, Ser.

Int. Cl. B05c 11/10 US. Cl. 117102 R 31 Claims ABSTRACT OF THEDISCLOSURE Coated paper is produced by applying a composition comprisinga polymeric material with or without added plgments dispersed in aliquid vehicle, freezing the coatmg, subjecting the frozen coating to avacuum to remove the majority of the liquid vehicle, and returning thedried or partially dried coating on the web to ambient temperature.

BACKGROUND OF THE INVENTION Conventional coated printing papers andother coated papers are made by applying a coating consisting ofinorganic pigments, binders and a vehicle such as water to a paper webby any one of several well-known methods. In all of these, the vehicleis evaporated from the liquid state to leave a solid coating on thesurface of the paper. This drying step has usually been accomplishedthrough the use of such techniques as high velocity air impingement, bycontact with heated drums or by subjecting the wet coated sheet toinfrared radiation, microwave radiation and the like. These conventionalprocesses are similar in that the vehicle, for instance, water, isremoved from the coating by transforming the liquid vehicle into vapor.When drying a coating from the substantially liquid state, the forcesarising due to the surface tension of the residual liquid cause thecoating film to shrink as the vehicle is removed such that upon more orless complete removal of the vehicle, the coating more or less conformsto the original surface of the web which was coated. This shrinkingeffect and the tendency of the coating to conform to the web surfaceduring drying has been a major reason for continual development effortstowards high solids coatings applied for instance by trailing bladetechniques, or towards drying the wet coating when in position against apolished drum, in attempts to obtain better printing surfaces. Despitethese developments, this shrinkage and conforming property produces asurface rough enough that the additional step of supercalendering isgenerally required to smooth the surface of the coating so that it isuseful in high quality printing applications. Typically, with the priorart procedures described above, the conversion of a paper base to a highquality printing paper is attended by an increase in apparent densityfrom an original to 12 pounds per 3300 sq. ft. per mil of thickness tovalues ranging from about 18 to 24 Ice pounds per 3300 sq. ft. per mil.As a consequence, it has become common in the art of printing papers toassociate heavyweight or high density with high quality.

Attempts have been made to improve the quality of presently availablecoated papers and to produce a high quality coated product without usingheavyweight paper and without increasing the thickness of the coatedsheet. Generally, such attempts, which have involved the formation ofporous type coatings prepared from two-phase emulsified oil in watersystems, have not been commercially successful to any great extent. Withsuch two-phase systems, the porous nature of the coating is the resultof forming globules of oil as a discontinuous phase in the liquidcarrier for the coating material. The production of pores in the finalproduct requires removal of this discontinuous oil phase generallysubsequent to substantial removal of the continuous phase portion of theliquid carrier. This removal procedure, however, increases the overallcost of manufacture of the coated product and does not eliminate furtherprocessing, such as supercalen-' dering or other smoothing operations,as is usually required with normal coating procedures to produce anacceptable high quality product. Additionally, the ability to formuniform pore sizes in the dried coating from these prior art two-phasecoating formulations is severely limited by the practical difiiculty offorming a uniform particle size emulsion coating formulation. Suchemulsions will generally have a broad distribution in particle size,which broadens further with the age of the formulation, temperaturechanges, etc. Consequently, in practice, a number of the resulting poresor voids will have dimensions either below or above the most useful sizerange for optimum scattering of visible light. Maximum opacifying andbrightening efiiciency is therefore difficult to obtain and difficult toreproduce. Further, this aging coalescence can readily occur duringdrying of the emulsion coating. It is not uncommon to have a void sizevariation as great as 50 fold.

In addition to a more or less random and broad distribution of voiddimensions, this prior art two-phase coating technology results in amore or less random distribution of the material surrounding thesevoids. Variations as great as fold are common. A physical configurationof this type will have little structural strength. The thin membraneswill break readily under compressive, shearing or printing forces. Thus,with the two-phase coating systems, smooth, printable coatings arediflicult to obtain without either adding pigments, using a subsequentsurface treatment, or overcoating. All of these treatments, however,serve to reduce the appearance, optical properties, lightweight and/oreconomical advantages which might otherwise be characteristic of thesecoatings.

In addition to the two-phase coating systems, plastic or resinouscompositions have been formed into porous films for application onto abacking material. The porous film is formed by first freezing adispersion or emulsion of the composition. The freezing producesexpanded ice structures in the mass; whereupon, subsequent thawingproduces a resinous mass which is generally sponge like. The thawed massis then broken down into a moldable or coatable composition. Thiscomposition can be applied to a suitable backing material; and with theapplication of heat, the dispersing medium is removed and thecomposition fused together into a porous film.

SUMMARY OF THE INVENTION The present invention provides a method whichpermits the direct production of a lightweight coating with highsmoothness and opacity without necessity for subsequent smoothing,coating or surface treating operations and without the necessity ofusing a cumbersome twophase liquid system. Briefly, the coated productis one which is formed by coating a web or sheet of paper with a mixtureconsisting of a polymeric material dispersed in a liquid vehicle with orwithout pigments and adhesives, freezing the applied coating before thevehicle has evaporated or been substantially absorbed into the paper,introducing the frozen coated sheet into a vacuum chamber where liquidvehicle is substantially sublimed away from the frozen coating, andreturning the substantially dried web to normal temperature.

We have found that the thickness of the dried coatings preparedaccording to this disclosure remain substantially the same as its wetthickness when freshly applied to the substrate. The surface of thecoating thus prepared has substantially the same high degree ofsmoothness and gloss as the wet coating at the time it was frozen. Theopacity of the resulting lightweight coating is markedly improved overan equivalent weight of the best known conventionally dried pigmentedcoatings. Further advantages of the coatings of this invention includelow density and high gloss. The dried coating thickness is alsosubstantially the same as the wet thickness.

The invention is particularly advantageous for the continuousmanufacture of lightweight opaque printing paper. Coated paper productsof this invention are readily made water insensitive, of high brightnessand of strength commensurate with that encountered in known conventionalprinting processes.

Freeze-drying as a process is well known and has been used in the foodindustry for a number of years. However, it has generally beenconsidered merely as a means for removing water from, for instance, afood product without damaging the material, that is, without destroyingits natural structure as would occur for instance by normal hightemperature drying processes. In fact, in the area of food technology,one of the desirable properties which has been obtained is ease ofsubsequent water recovery into the solid when the product is later to beused, that is, water could be restored to the solid rapidly because theresulting product was extremely porous. Of course, this water-regainproperty is highly undesirable in coated papers and we have therefore,developed coatings which attain a high degree of water resistance, eventhough dried by the freeze-drying process. These coatings have inaddition other desirable properties as described herein. It has, forinstance, been found that a high degree of opacity can be obtained inthin films from virtually any normally solid polymeric materialdispersed in a liquid vehicle provided that the vehicle can beconveniently frozen and subsequently sublimed away from the polymericmaterial. The reasons why greater opacity is obtained according to thisinvention are not completely understood but it is believed that theopacity results from the relatively uniform void size and intervoidspaces left by removal of the solid vehicular crystals from the frozencoating, and that the degree of opacity is a function of the voidvolume, the relatively narrow void size distribution and the index ofrefraction of the polymeric material employed.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic view showingthe components of the processing equipment for carrying out the processof this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS There are literally hundreds ofpolymeric materials and vehicles that can be used in this invention andall of these cannot conveniently be listed here. The primary limitationto the operation of our process is that the vehicle in the coatingformulation can be frozen and removed by sublimation from the solidcoating formulation. Two practical limitations can be stated asadvantageous but not as in- 4 herently limiting to the practice of thisinvention. Thus, the polymer liquid vehicle combination should haverheological properties suitable for coating application processes knownin the art and sufficiently high solids should be obtainable withinappropriate viscosities to avoid undue expense for removal of the liquidvehicle. It is particularly advantageous to work with aqueous solutionsor aqueous suspensions.

The polymeric materials that can be used according to this inventioneither alone or in admixture include all polymeric materials capable ofbeing coated on a substrate such as paper and particularly those used inthe paper industry and well known to those skilled in the art.

Examples of polymeric materials that appear particularly advantageousinclude starch, dextrine, casein, soya protein, polyvinyl alcohol,regenerated cellulose, poly (styrene/maleic anhydride), poly(styrene/butadiene), acrylic polymers and copolymeric mixtures thereof,and the like.

In the practice of this invention, it is preferred to utilizeformulations at a solids level between 5% and 60% by weight and evenmore preferably between 15% and 40% by weight in a single phase liquidcarrier.

Organic and inorganic pigments which are well known in the paper coatingart may be added to the formulation in essentially any proportion forspecial effect such as color, but some special opacifying pigments suchas rutile titanium dioxide actually decrease the opacifying power of thefreeze-dried coating when compared on a pound for pound basis with asimilar coating not containing the pigment. Except as indicated herein,the coating formulation need not be treated physically or chemically inany unusual way prior to its application to the web.

Pigments are included in conventional paper coatings not only toincrease opacity and whiteness of the coating, but also to providemicroporosity necessary to trap printing ink, thereby preventing setolffrom one sheet to another during printing, and to allow adequate inkpickup during printing to obtain desired image density. Examples ofpigments which can be used include coating clays, zinc oxide, titaniumdioxide and satin white. The freeze-dried coatings according to thisinvention contributes similarly to these aspects of the printing processand produce printing results superior to conventional coatings.

It might be anticipated that the dried porous or cellular coatings ofthis invention would have little or no measurable strength and indeed ithas been found that unless proper steps are taken, this effect isobserved. Various procedures can be employed, however to impartadditional or the necessary strength to the coatings prepared by thefreeze-drying technique. These methods should be construed asinstructive to the practice of this invention rather than as limiting toit.

One method for assuring adequate strength in the final coating involvespreparing formulations consisting of two or more dissimilar polymericmaterials. The freeze-dried coatings prepared from such systems exhibitunexpected strength. Although the exact reasons for this unexpectedstrength are not known, it is believed that these systems obtain theirstrength by virtue of the fact that freezing does not actually occurinstantaneously regardless of how fast the coating appears to have beenfrozen. We have postulated that certain molecular regions within thecoating are probably occupied by dissimilar functional groups frompolymeric molecules and that some water molecules do not become frozeneven at apparent ambient temperatures as low as minus C. Hence, whenthese water molecules are removed, sufiicient residual surface tensionexists which forces the group of polymeric molecules involved in thatregion into suificiently close contact that bonding can take placebetween them even though the greater part of any one molecule may besubstantially free of this intermolecular bonding.

Various dissimilar polymers, i.e. those having dissimilar functionalgroups such as in Example VII, can be used according to this inventionto obtain sufiicient strength in the coating for printing purposes.

Another method for achieving strength in freeze-dried coatings involvesfreezing the coating slowly and limiting the final temperature to whichthe frozen coating is subjected, that is maintaining the temperature ashigh as possible and yet retain a frozen coating. Whether or notdissimilar polymer materials are used, this approach accomplishessubstantially the same effect as the immediately preceding method. Thus,it is believed that all of the polymeric molecules would be associatedwith sufiicient liquid vehicle molecules to permit good intermolecularbonding while the crystals of vehicle which have been allowed to grow tothe desired size provide the void spaces which contribute to theexceptionally high opacity characteristics of the freeze-dried coatingsmade accord ing to this invention.

Another technique involves the addition of an emulsified thermoplasticor heat fusible material to the coating formulation, for instance, inthe case of an aqueous system, an acrylic emulsion polymer or a waxemulsion respectively. In this case, after the coating has been freezedried, it is subject to a source of heat suflicient to cause fiow of thethermoplastic or fusible material, thereby effecting substantialintermolecular bonding. Such coated products when subsequently returnedto normal temperatures have the strength required of a printing coatingas illustrated in Example XVI. It should be apparent that the majorpolymeric ingredient of the coating must not have thermoplastic orfusion properties similar to the ad ditive just described. Differentfunctional groups are not required using this technique.

Another technique of imparting strength to a freezedried coatingaccording to this invention, involves subjecting the coating to a bondforming chemical reaction after the coating has been dried wherein atleast one part of the reacting system is added to the coating after ithas been dried. In this case, it is desirable to limit the quantity ofreactive material taken up by the dried coating so as not to impair theinherent opacity of the coating. Typical reaction system might includevapor phase isocyanate reactions, epoxy reactions, carboxyl, amineformaldehyde,

and hydroxy reactions, polymerization of ethylenimine,

or other well known chemically reactive polymeric or adhesive systemssuch as that set forth in Example V.

The processing equipment required for practice of this invention isunique in that the various components necessary to the invention have toour knowledge, never been assembled into one apparatus, despite the factthat each of the major components exist and are well known to thoseschooled in the art of coating, vacuum treatment of webs, and freezedrying. That is to say, the process equipment necessary to thisinvention is made up of a series of already known operational units. Theoverall process is described in the following paragraphs.

Although this invention may be carried out on a batch basis in which aroll of paper would be loaded into a vacuum chamber wherein the entireprocess is carried out and then the finished product unloaded, ourinvention is advantageously directed toward the use of a continuous orair-to-air process. The operational units required for the continuousprocess are an unwind stand, a coating application unit, a freezingunit, a vacuum chamber unit, a heating and condensing system connectedto the vacuum unit, and a subsequent windup stand. These process unitsare indicated schematically in the drawing,

It should be obvious that iteins such as unwind stands, windup stands,and various continuous web conveying and splicing devices which are wellknown can be employed and these need no further comment. The coatingunit may be selected from any of the types which can apply fluids ofsuitable rheology so as to obtain desired coat weight within desiredtolerances when applied to a paper web.

For example, trailing blade, inverted blade, air knife, roll coating,reverse roll coating, wire wound rod doctor, extrusion or any of theother well-known techniques for applying coatings to a web may beemployed. This list is intended to express the breadth and scope of ourinvention and not to limit the applicable techniques to those named.

Likewise the freezing step may be carried out by any one of severaltechniques. A blast. of cold air or other gas may be used, the sheet maybe brought into contact with the cold surface, such as for instance, thechilled backing roll for a trailing blade, or a large chilled drum roll,or the sheet may be dipped into a cold immiscible non-wetting fluid,each of which is known in various segments of industry which utilizefreezing as a process operation.

Equipment appropriate to leading a web of material from atmosphericconditions into and subsequently out of a reduced pressure chamber in acontinuous or semi-com tinuous manner has been developed for vacuummetallizing of strip metals, glass, plastics, and paper. Similar unitscan be advantageously employed in the practice of our invention.However, in the case of freeze-drying, the mechanical problems are lesssevere since the pressure used in the vacuum chamber can be orders ofmagnitude higher than that required for vacuum metalizing. Thus, apressure of 50 to 500 microns of mercury is adequate for freeze-dryingprocesses whereas vacuum metallizing frequently requires pressures aslow as 0.5 micron of mercury.

This portion of the process involves passing the web through forinstance, rotary seals or narrow slits into succeeding chambers at lowerpressure. Each chamber can be individually pumped and can be of very lowvolume. The number of such entry and exit chambers required will dependupon the efiiciency of the rotary seals and the capacity of the pumps.Once the coated sheet reaches the main chamber in which the ambientpressure is maintained below the sublimation pressure of ice at thetemperature of the sheet, the drying process begins. The rate ofsublimation can be accelerated by supplying heat to the sheet at a rateequal to the rate of heat usage in sublimation. This heat may besupplied in a number of ways, for instance, by use of microwave ordielectric heaters, infrared radiation, electrical resistance through aconductive material and many others. It is important, however, duringthe sublimation process that the coating not be grossly melted therebydestroying the porous structure. To improve the rate at which water canbe removed from the chamber, it is desirable to condense the water vaporout of the vacuum chamber before the vapor gets to the vacuum pumps.This is ordinarily accomplished by interposing heat exchangingcondensers in the vacuum line prior to the vacuum pumps. The heatexchangers are maintained at a temperature well below the ice pointcharacteristic of the operating pressure, which temperature isconsequently also below the temperature of the coated web. A second oralternate heat exchanger can also be provided so that the ice buildup onthe primary heat exchanger may be removed without interrupting thecontinuous operation.

After the web has been dried in the vacuum chamber, it is led out of thechamber through a similar set of seals, given a further heat or chemicaltreatment if desired, then rewound into a roll. The same process canserve for a sheet which has been coated on either one side or two, withonly minor modifications to the geometry required for a two-side coatedsheet as will be recognized by those skilled in the art.

With proper mechanical design of the coating application system andentry seals to the vacuum chamber, the wet coated sheet could be leaddirectly into the vacuum chamber without necessity for first freezingthe coating. The overall process remains similar in this case, since thecoating will freeze rapidly in the vacuum due to the cooling effect ofthe vaporization. Once frozen in this man- 7 ner, the process continuessubstantially as we have already described. This is illustrated by theExample XVII disclosed herein.

The coatings that can be used according to this invention can compriseconventional paper coating compositions having solids contents in theusual and conventional range of coat weights or wet thicknesses aspreviously practiced in the paper industry. With respect to wetthickness of the coating, it is significant to point out that thecoatings according to this invention have the same thickness both wetand dry, while conventional air drying of these same coatings reducesthe thickness of the wet coatings. The invention is of course notlimited to paper but can be practiced on various substrates, such asplastic films, particularly where the applied coating is to be printed.

This invention can be more fully understood by reference to thefollowing examples.

EXAMPLE I A sheet of paper rawstock having a basis weight 44 pounds per3300 sq. ft. was placed on a metal plate cooled by Dry Ice. A coatingformulation consisting of aqueous ammonium caseinate, at 15% by weightsolids was applied to the sheet by means of a Bird Film Applicator at arate of 2 pounds of solids per 3300 sq. ft. and frozen immediately. Thepaper was placed in a vacuum chamber and evacuated to 500 microns andheld there until the frozen coating was dried by sublimation and thenreturned to ambient conditions. The drying only took about 2 minutes.Casein, freeze dried in the above manner forms an opaque, white coatingwith an exceptionally smooth, glossy surface. The properties of thepaper were considerably improved by the coating as noted below:

2 lbs. C 1 S sheet stock 78 82. 5 85 90 65 12 Hunter gloss 8 66 Thefreeze-dried, casein coated paper was printed on a Diamond Gardnerrotogravure press and produced sharp images superior to those producedon a commercial grade of paper designed for rotogravure printing.

EXAMPLE II EXAMPLE III A coating formulation containing 80% casein and20% of paraffin wax emulsion (Paracol 404 g.) at 15% total solids wascoated on paper and freeze dried as in Example I. The film was white andopaque and after a heat treatment at 100 C. for 1 minute wassubstantially water resistant.

EXAMPLE IV A coating formulation consisting of 68% casein and 32% of a55% solids styrene/butadiene copolymer (60/ 40) latex, marketed byKoppers under the name Dylex K-55, was coated at 15% total solids on apaper web as in Example I. This coating was white and opaque and moreresistant to water than the coating of Example I.

8 EXAMPLE v A coating formualtion consisting of 78% casein, 19.5% ofmethylated methylol melamine-formaldehyde resin, marketed by Monsantounder the trade name Scriptite 31, and 2.5% diammonium phosphate wascoated on paper at 15% total solids and frozen as in Example I. Thefreeze-dried coating was then cured for 20 seconds at 180 C. The filmproduced was white, opaque and water resistant.

EXAMPLE VI Soya protein was substituted for casein in Examples I-V andvery little difference was noted. The dark color of the protein howeverdecreased the brightness by several points relative to Examples I-V.

EXAMPLE VII A coating formulation consisting of 68% dextrine and 32% ofa partial ester of a maleic anhydride-styrene copolymer, marketed byMonsanto Chemical Company under the trademark Scripset 550, was appliedand freeze dried at 23% total solids as in Example I. The coating waswhite, opaque, and water resistant. The Shefiield smoothness was 10 andthe Hunter gloss measured which indicated a smoother surface than wasattained using casein or protein.

EXAMPLE VIII A coating formulation containing HT clay and 20% oxidizedstarch at 40% total solids was coated on paper and freeze dried as inExample I, and compared to a conventionally air-dried sheet with thesame coating and coat weight. Freeze-drying improved the brightness from80 to 86, the opacity from 90 to 94, and tripled the Bekk smoothness 80to 2140.

EXAMPLE IX A coating formulation containing 80% HT clay and 20%polyvinyl alcohol at 20% total solids was coated on paper and freezedried as in Example I and compared to an air-dried control with the samecoat weight. The brightness on the control was 77 and the opacity 87.The freeze-dried sheet had a brightness of 85.5 and an opacity of 92.5.

EXAMPLE X The coating formulation described in Example VII was appliedas in Example I at 23% total solids and put in the vacuum chamber andkept frozen while the chamber was evacuated. At 1000 microns totalpressure the sheet was removed from the cold metal support and suspendedparallel to an infrared heater in vacuum which was turned on at a wiretemperature of 1200 C. for 10 seconds. The chamber was then returned toatmospheric pressure and the coating was found to be dry. The total timethat the paper was in the vacuum was 2 minutes. The coat weight wasmeasured at 5.5 lbs, per ream and the gloss and smoothness correspondedto those measured in Example VII.

EXAMPLE XI An aqueous casein coating formulation at 15% solids wasapplied to paper rawstock and allowed to penetrate for 1 second beforefreezing on a cold metal plate and freeze-drying as in Example I. Thecoating produced had a fiber tearing bond to the paper and similarproperties described in Example I.

EXAMPLE XII A coating formulation consisting of oxidized starch (StaycoC) at 15 solids was applied as in Example I and freeze dried. A 5.4lb./ream coating raised the papers brightness from 79 to 88 and itsopacity from to 94. The sheet was smooth and glossy.

9 EXAMPLE XIII An aqueous formulation containing 100 parts of ammoniacut Polish casein and 20 parts of Azite (Azite 900 Liquefier, AmericanCyanamid Co.) at 24% solids was applied to a 42-pound per 3300 squarefoot paper base, immediately frozen by a 1-2 second contact of the sheetwith a glass plate previously cooled by contact with Dry Ice, placed inan air tight chamber connected with a vacuum pump and the frozen waterremoved from the coating by vacuum sublimation. The characteristics ofthe resulting dried, coated paper are given below.

Density of base stock 1 13 Weight of base stock 42 Weight of coating 26-7 Density of product 1 12-13 Caliper of product (mils) 4.0-4.5 Weightof product 2 48-49 Opacity 95-97 Brightness 88-90 Smoothness 13-23Olfset printability Good 1 Pounds/3300 sq. ft. per mil of thickness. 9Pounds/3300 sq. ft.

The product of Example XIII is smoother than conventionally coated,air-dried, supercalendered products. The apparent density of ExampleXIII coating is substantially the same as that for its base stock, whichcompares favorably with conventional products where the product densityis typically doubled to achieve product properties comparable to thoseof Example XIII.

EXAMPLE XIV An aqueous formulation containing 75 parts of ammonia cutPolish casein, 7.5 parts of Azite and 200 parts of HT clay (kaolinite,:Edgar Clay Company), at 41.4% solids was applied to a 34 /2 pounds per3300 square feet paper base, immediately frozen and vacuum dried asdescribed in Example XIII. The characteristics of the resulting dried,coated sheet are given below:

Density of base stock 14 Weight of base stock 34-35 Weight of coating6-8 Density of product 13-15 Caliper of product 2.8-3.0 Weight ofproduct 40-43 Opacity 93-96 Brightness 87-88 Smoothness 12-24 Offsetprintability (1) Fair-good.

A coating formulation containing 100 parts of enzymeconverted starch,600 parts of No. 2 coating clay and 100 parts of calcium carbonate(Purecal 0, Wyandotte Chemical Co.), dispersed in 600 parts of water to58% solids was applied in a manner similar to that described in ExampleXIII. The dried product characteristics are listed below:

To apply the coating composition of Example XV to a paper in theconventional manner with supercalendering would require twice thecoating weight to obtain opacity and brightness values approaching thoseof Example XV. Also, higher densities; would be required.

EXAMPLE XVI An aqueous coating formulation containing by weight parts ofa 15% ammoniacal Polish casein dispersion, 32.6 parts of an acrylicemulsion polymer (Rhoplex AC-73, Rohm and Haas Company), 2 parts of acondensation product of melamine and formaldehyde, marketed by AmericanCyanamid Company as 'Parez'613 and 0.64 part of a 25% ammonium chloridesolution was freeze dry coated in a manner similar to Example I. Thedried coated paper was then heated for 30 seconds at C. Strengthimprovement was noted by a marked increase in resistance of the coatingto marking by a metal stylus.

EXAMPLE XVII An aqueous coating formulation containing 100 parts byweight of casein, 25 parts of a styrene-maleic anhydride copolymermarketed by Monsanto Chemical Company as Scripset 54 and 25 parts of asynthetic rubber emulsion polymer marketed by E. I. du Pont, Inc. as aNeoprene emulsion, was applied to a paper base, placed in a vacuumchamber and evacuated to about 250 microns and held there until thecoating was dried, re- Sliltd in an opaque coating having a brightnessvalue 0 84.

The coated products produced according to the above examples havevarious degrees of strength depending upon the particular process usedas discussed above, but all are useful and the particular method chosenwill depend upon the particular end use intended for the coatedsubstrate. Dilferent printing processes, for example, require differentcoating strengths or tack strength as is well known in the art. Thecoated substrates can of course be used for purposes other than printingwhere higher opacity, brightness, and smoothness are desired.

The coatings of this invention allow greater latitude in the design ofprinting paper products than heretofore possible. Thus, rawstocks havingbase weights in the range from about 15 to 80 pounds per 3300 sq. ft.can be used, with or without added pigments or fillers to manufactureproducts of essentially nonvarying, low apparent density but rangingfrom about 3.0 to 6.0 mils in caliper. Thus, coatings of this inventionhaving substantially greater thicknesses than the prior art coatings canbe used to produce coated products having total thicknesses comparableto prior art products.

With the coated product of the present invention, fiber showthrough ofthe undnerlying base material is virtually eliminated. This is sobecause the coating lies essentially completely on the paper surface andthe surface of the coating is well above the uppermost fibers of thepaper base. With the prior art coatings, fiber showthrough often occursdue to shrinkage during conventional high temperature drying. Thisnon-uniformity, smoothness defect in the coating surface can causesubsequent printing problems which are not exhibited with any of thecoatings described above.

From the above examples, it is seen that coatings produced in accordancewith the teachings of the present invention are uniform in thickness andmay range from about 0.1 to 10 mils. Also, coatings of the present invention are chaarcterized as having anhywhere from 20 to 90% void volumewhere the void volume is made up of numerous cells which may range from0.1 to microns in size. However, we have found that in any particularcoating, the size of the voids advantageously varies no more than aboutfour fold. Furthermore, the intervoid thickness of the coatings of thepresent invention Will be substantially uniform to form a continuousmatrix surrounding the voids. The intervoid distances may vary from 0.1to 5 microns; with these distances in any particular application varyingonly from about two to four fold.

When the coatings of the present invention are applied to paper, thedensity of the coated product is substantially the same as that of thebase material to which the coating is applied. More particularly, thedensity of the coating may range from 0.06 to 1.2 grams per cubiccentimeter with the density of the coated product being no greater than20% more than the density of the original base material. Thesecharacteristics of the coated product of the present invention comparefavorably with products produced according to the prior art techniqueswhere thicker and denser coatings are required to produce acceptablehigh quality paper.

Paper products coated in accordance with the teachings of the presentinvention generally have TAPPI opacity values ranging from 90 to 100,standard brightness values ranging from 80 to 100, Shefiield smoothnessvalues ranging from about to 50, apparent density values from about 10to 18 pounds per 3300 sq. ft. per mil of product thickness, good toexcellent printability and good marring and crush resistance.

From the foregoing examples, it is apparent that the present inventionoffers a technique of producing a high quality lightweight opaqueprinting paper surface having gloss, smoothness and inherentprintability qualities so that the usual subsequent operation ofsupercalendering may be entirely eliminated. Elimination of thesupercalendering step reduces the need for capital and labor in themanufacture of coated papers and eliminates a high wastage of productand time normally associated with the supercalendering step.

It will also be apparent from the foregoing disclosure that thepolymeric materials for use in our coating compositions can be selectedfrom a wide variety of natural polymers such as casein and starches andtheir chemical modifications, as well as addition-type polymers andcopolymers of vinyl chloride, acrylonitrite, styrene, acrylic andmethacrylic acid and their polymerizable derivatives, ethylene,butadiene, vinyl acetate, fiuorinated monomers and the like, and alsofrom condensation polymers such as polyesters, polyamides,phenol-formaldehyde resins and the like, provided that the dried residueof these polymeric compositions is a solid, more or less permanent masssubstantially as illustrated by the examples in this disclosure and thatthe composition can be dissolved, dispersed or otherwise suspended in aliquid medium which can be frozen and sublimed under reduce pressure inaccordance with our disclosure.

We claim:

1. The process which comprises:

(a) coating a paper web with a coating comprising a polymericcomposition contained in a liquid carrier;

(b) freezing the applied coating by reducing its temperature to belowthe freezing point of the applied coating to solidify the coating;

(c) removing a sufficient amoun of the frozen liquid carriersubstantially as a vapor directly from the solidified coating underreduced pressure that the coating will remain solid and substantiallydry when the web is returned to atmospheric pressure and 12 ambienttemperature and will possess a thickness substantially the same as thatof the coating when first applied; and

(d) returning the frozen web to ambient pressure and temperatureconditions.

2. The process according to claim 1 in which:

(a) thermal energy is applied to the frozen web under reduced pressure.

3. The process according to claim 2 in which:

(a) thermal energy is applied to the frozen web under reduced pressurein the form of infrared radiation.

4. The process according to claim 2 in which:

(a) thermal energy is applied by microwave or dielectric heating.

5. The process according to claim 2 in which:

(a) thermal energy is applied by conduction from a heated contactingsurface.

6. The process according to claim 1 wherein:

(a) the liquid vehicle is water; and

(b) the polymeric composition is selected from at least one member ofthe group consisting of starch, oxidized starch, dextrine, casein, soyaprotein, polyvinyl alcohol, styrene-butadiene copolymers, emulsifiedwax, and partial esters of styrene-maleic anhydride copolymers.

7. The process according to claim 1 wherein:

(a) the liquid vehicle is water; and

(b) a mixture of polymeric composition is used in which:

(1) one is selected from at least one member of the group consisting ofstarch, oxidized starch, casein, soya protein, dextrine or polyvinylalcohol, and

(2) the second is selected from at least one member of the groupconsisting of polymers of melamine-formaldehyde or urea-formaldehyde,partial esters of styrene-maleic anhydride copolymers, styrene-butadienecopolymers and emulsified waxes.

8. The process of claim 1 wherein:

(a) the Web is subjected to the further heat treatment after coming outof the vacuum chamber to attain added strength or added strength andwater resistance.

9. The process according to claim 8 in which:

(a) the liquid vehicle is water; and

(b) the polymeric composition is selected from at least one member ofthe group consisting of starch, oxidized starch, dextrine, casein, soyaprotein, polyvinyl alcohol, styrene-butadiene copolymers, emulsifiedwax, or partial esters of styrene-maleic anhydride copolymers.

10. The process according to claim 8 in which:

(a) the liquid vehicle is Water; and

(b) a mixture of polymeric materials is used in which:

(1) one is selected from at least one member of the group consisting ofstarch, oxidized starch, casein, soya protein, dextrine and polyvinylalcohol, and

(2) the second is selected from at least one member of the groupconsisting of pre-polymers of melamine-formaldehyde prepolymers ofureaformaldehyde, partial esters of styrene-maleic anhydride copolymers,styrene-butadiene copolymers, and emulsified wax.

11. The process according to claim 1 wherein:

(a) the solvent is benzene; and

(b) the adhesive is at least one member of the group consisting ofpolystyrene and a cross-linkable styrene copolymer.

12. The process of claim 1 in which:

(a) the liquified composition coated on the web contains an inorganicpigment.

13. The process of claim 1 in which:

(a) up to about 60 percent by weight of the polymeric 13 composition isreplaced with inorganic pigments selected from at least one member ofthe group consisting of coating clays, zinc oxide, titanium dioxide, andsatin white.

14. The process of claim 1 in which:

(a) the coating of the Web contains a polymeric composition chemicallyreactive with at least one member of the group consisting of isocyanate,epoxy, carboxyl, amine, hydroxy containing compounds and formaldehyde;and

(b) contacting the coated web containing the chemically reactivepolymeric composition after it has been removed from the vacuum chamberand dried with at least one member of the group consisting ofisocyanate, epoxy, carboxyl, amine, hydroxy containing compounds andformaldehyde in the vapor phase to impart added strength and waterresistance to the coated web.

15. The process which comprises:

(a) coating a base material a paper web with a coating comprising apolymeric composition contained in a single phase liquid carrier;

(b) solidifying said liquid carrier at discrete locations throughoutsaid coating; and

(c) removing the liquid carrier in its vapor state from said coating.

16. The process according to claim 15 in which:

(a) the coating is frozen to elfect the solidification of said liquidcarrier.

17. The process according to claim 16 wherein:

(a) the liquid carrier is removed by sublimation.

18. A coated product comprising:

(a) a base material comprising a paper web, and

(b) a thin coating of a polymeric composition adhered to said basematerial and comprised of (l) the dried residue of a frozen dispersionof said polymeric composition in a liquid carrier.

19. A coated product comprising:

(a) a base material comprising a paper web; and

(h) a coating of polymeric composition adhered to said base material andcomprised of:

(1) the dried residue of a wet coating consisting of a dispersion ofsaid polymeric composition in a single phase liquid carrier, said driedresidue having a thickness substantially the same as that of said wetcoating.

20. A coated product comprising:

(a) a base material comprising a paper web; and

(b) a coating of a polymeric composition adhered to said base materialand comprised of:

(1) the dried residue of a dispersion of said polymeric composition in asingle phase liquid carrier, said residue having voids spacedtherethrough at discrete locations previously occupied by said liquidcarrier prior to drying of said coating.

21. A coated product according to claim 20 wherein:

(a) said coating contains an inorganic pigment.

22. A coated product according to claim 21 wherein:

(a) said inorganic pigment is selected from at least one member of thegroup consisting of coating clays, zinc oxide, titanium dioxide, andsatin white.

23. A coated product according to claim 20 wherein:

(a) the polymeric composition is selected from at least one member ofthe group consisting of starch, oxidized starch, dextrine, casein, soyaprotein, polyvinyl alcohol, styrene butadiene copolymers, emulsifiedwax, and partial esters of styrene-maleic anhydried copolymers.

24. A coated product according to claim 20 wherein:

(a) the polymeric composition consists of dissimilar polymericmaterials.

25. A coated product according to claim 24 wherein:

(a) one of the polymeric materials is one selected from at least onemember of the group consisting of starch, oxidized starch, casein, soyaprotein, dextrine or polyvinyl alcohol; and

(b) another of the polymeric materials is one selected from at least onemember of the group consisting of polymers of melamine-formaldehyde orurea-formaldehyde, partial esters of styrene-maleic anhydridecopolymers, styrene-butadiene copolymers and emulsified waxes.

26. A coated product according to claim 20 wherein:

(a) the apparent density of said product is no more than about 20%greater than that of said base material.

27. A coated product according to claim 26 wherein:

(a) the apparent density of said product is substantially the same asthat of said base material.

28. The product according to claim 26 wherein:

(a) the apparent density of said coating is from between about .06 to1.2 grams per cubic centimeter.

29. A coated product according to claim 20 wherein:

(a) said voids are uniformly spaced throughout said coating.

30. A coated product according to claim 29 wherein:

(a) said voids vary up to about four fold in size.

31. A coated product according to claim 30 wherein:

(a) the intervoid distances of said coating vary from about two to fourfold.

References Cited UNITED STATES PATENTS 3,245,151 4/1966 Eichmanns 34-53,403,046 9/1968 Schlacke et al. 117-1 19.2 X 3,376,158 4/1968 Buser117-1192 3,228,786 1/ 1966 Fitzgerald et a1. 117-62 3,428,584 2/ 1969Riley 260-15 3,349,749 10/ 1967 Utschig 117-119.2 X 2,668,364 2/1954Colton 34-5 X 2,897,094 7/1959 Hayes et a1. 117-62.1

WILLIAM D. MARTIN, Primary Examiner M. R. LUSIGNAN, Assistant ExaminerUS. Cl. X.R.

34-5; 117-119, 119.2,, 119.6, UA, 155 L, 156, 158

